The traditional "wet" chemistry techniques in analytical chemistry and its more sophisticated progeny, clinical chemistry, have in recent decades been replaced by electronic instrumentation. With the advent of instrumentaton, accuracy in reproducibility of experimental measurements has been enhanced. Such accuracy is of particular importance in clinical chemical techniques and of the greatest importance in biomedical measurements where minute (part per million) measurements are common. Linking such instrumentation and automated processing with the microprocessors and affordable computer technology, has resulted in another step in the evolution of analytical and clinical chemical techniques.
In the sub-discipline of electro-chemical measurements, great advances in such instrumentation have been made. Generally, conventional electro-chemical measurements require the measurement of two sample solutions containing two different known concentrations of a substance for calibration purposes followed by measurement of a solution containing an unknown quantity of the species. Electro-chemical methods generally require use of a reference electrode, a substance specific electrode and a bridge between the solution in order to achieve a cell for potentiometric measurements. The electrical signal (commonly in millivolts) obtained from the cell is proportional to the ionic activity and, therefore, concentration of the substance in the solution. The signal/concentration relationship is algebraically expressible by a Nernst equation EQU V=M.sub.f [C]+I+J (1)
where
V is the voltage (signal) PA1 M.sub.f is the slope (a constant for the particular electrode and substance) PA1 I is a constant for a particular substance PA1 J is the junction potential of the cell PA1 [C] is the ionic activity (concentration of the substance).
In order to establish the values necessary to solve the equation, it is first required to determine the slope, M.sub.f, for the electrode. For this step, measurement of two solutions containing known concentrations are taken, the values inputted into the above equation and the equations solved simultaneously to obtain the slope. Next, it is necessary to determine the constant, I, for the particular substance in solution relative to the particular electrode. The junction potential is also determined by conventional methods. The foregoing techhique is commonly referred to as a double or dual point calibration. Recent developments in electrode technology have dispensed with the need for the slope determination by providing preset, one-shot electrodes where the slope is known for a particular substance and electrode structure. These devices are generally limited, however, to one time use due to the slope shift after a period of exposure to solution. Slope shift is attributable to, among other causes, hydration of a previously unhydrated electrode. In view of this arrangement, such one shot electrodes are confined to use with specific systems and particular electrode arrangements.
Great improvements have been made in the sensitivity of sensors employed for electro-chemical analysis. Many relatively new sensor types have now found their way into the laboratory. Most notable are variants of the ion selective electrode (ISE), the enzyme base selective electrode (EBSE), the anti-body based selective electrode (ABSE), chemical field effect transducer (CHEMFET) and the ion selective field effect transducer (ISFET). Each of these sensor types may be incorporated into a number of physical variants including coated wire electrodes, thin film electrodes, etc. These are employable, not only for clinical chemical application, but also for general use in such fields as industrial chemical, pharmaceutical, biochemical, environmental control, etc. Moreover, these devices now provide the technician with a considerable selection of devices and techniques which function to produce electrical signals proportional to the ionic activity of a particular substance or substances for which the sensors are specifically designed and, therefore, increasingly precise measurements.
Referring briefly to optical sensors and analytical methods primarily relying on colorometry, they, too, have experienced a corresponding rapid evolution. Significant advances are pronounced in the biochemistry field, e.g. enzyme and antibody-antigen reactions.
However, the technician is now faced with an increasing array of problems associated with the new technologies. For example, due to the sensitivity of the above-mentioned sensors, they may possess a bulky design. Notably, electro-chemical sensor systems generally require a reference electrode and species sensitive electrode, both of which must be carefully calibrated or preconditioned. Also, especially in the case of reference electrodes, supposedly identical electrodes may differ slightly due to manufacturing tolerances which can lead to erratic measurements, "drift" problems and junction potential errors.
Measurement variations may occur from use of such electrodes due to signal drift and varying junction potentials between the reference electrode in the media being studied and the associated electrodes. Junction potential contribution to the signal not only results from electrode structure, but also varies from instrument to instrument as well as measurement to measurement. For sensitive measurements, such variations are wholly unacceptable. Further problems are augmented by increasing sensitivity of the electrodes, particularly in biomedical applications, where precise measurements are critical. Factors such as the longevity, stability and contamination of the reference electrode, particularly when employed in hostile environments such as invasive monitoring during surgical procedures, must be accounted for, and have, thus far, escaped resolution. Finally, in devices requiring the relatively bulky reference electrode, electro-chemical systems have, for the most part, belied miniaturization.
During electro-chemical measurements of complex solutions (multicomponent) another problem arises, namely, separation of the signal from reference electrode junction potential. For measurement of complex solutions containing many potentially interactive electroactive species, in contrast to elementary assessment of a single species solution, the coefficient of activity (contribution by individual components) will defy precise determination due to electro-chemical synergy. Hence, the electro-chemical measurements of complex organic solutions, such as blood, necessitate interpretation of the signal due to the lack of precision in identifying the contribution of a particular targeted substance. Where precise measurements are required, the ambiguity stemming from such interpretation is, at best, risky and, at worst, lethal. Moreover, the contribution of drift by both the reference electrode and the specimen electrode coupled with the junction potential identification problem, could lead to anomalous measurements.
Moving now to a practical problem associated with prior art systems, it is the manufacture and supply of both species specific electrodes and reference electrodes. Generally, the reference electrodes are of a more sophisticated construction in order that they be reusable. Without more, it is evident that measurements using such electrodes would differ in every instance due to manufacturing tolerances. Accordingly, not only are the drift and calibration problems extant, but, also, standardization is difficult especially when conducting the several measurements of different solutions required for calibration and unknown solution evaluation.
Most analytical systems are exposed to the ambient environment. They are not anaerobic. An anaerobic environment is desirable, first, to more closely match in vivo conditions. Furthermore, it is important, for example, in blood gas analysis to avoid sample contamination from air so as to avoid skewing the results. Lastly, to obtain a series of substance measurements from a sample, requires considerable time and many individual measurements. Not only is the time factor detrimental but, also, specimen contamination and chemical changes in the specimen are likely to occur. Hence, it is desirable to maintain an anaerobic measuring environment to achieve accurate measurements of certain substances, and most notably, blood gas concentrations. Lastly, most known systems do not contemplate fixing or providing fixed volume delivery. Elaborate stirring or mixing arrangements are used to insure uniform transport to the sensor. It would be desirable to conduct measurements of a fixed volume of solution and especially desirable to provide analysis requiring only a small volume of solution uniformly delivered to the sensor to make the measurements.
Other practical considerations arise relative to laboratory use by the clinician. In the event that a system is intended to be reusable, it is incumbent upon the operator or technician to insure that the electrodes are not contaminated when preparing for a test. Thorough cleaning and recalibration is necessary for each use. Such efforts require considerable labor and render cost ineffective the use of reusable systems especially in hospital laboratories, etc. Where disposable systems are employed, problems arise relating to the technician's techniques.
Another aspect of electro-chemical apparatus that has escaped development is a compact, simply employable, field or laboratory use instrument which can be operated by persons having a minimum of skilled training. Miniaturized and standardized equipments are not available for providing analytical electro-chemical measurements like those described above.